Higher substrate ratios of ethanol to acetate steered chain elongation toward n-caprylate in a bioreactor with product extraction. Angenent Lab Substrate Ratio Carboxylate Platform

Anaerobic reactor microbiomes are capable of converting organic wastes to medium chain carboxylates via a process termed reverse beta-oxidation. Generally, an electron-rich substrate, such as one containing ethanol or lactic acid, is used to drive the chain elongation of shorter chain carboxylates, such as acetate and n-butyrate, to longer chain carboxylates, such as n-caproate or n-caprylate. Recent research has looked at coupling syngas fermentation, which produces principally ethanol and some acetate, to chain-elongation bioreactors to upgrade the produced ethanol to n-caproate or n-caprylate. Here, we examined the effect of different ethanol-to-acetate substrate ratios on the production of medium chain carboxylates. Higher ethanol-to-acetate substrate ratios led to higher selectivity for n-caprylate. The highest n-caprylate selectivity in this study occurred when the substrate contained primarily ethanol (ethanol-to-acetate ratio > 200, on a COD basis), however, a large fraction of the fed ethanol was not consumed and was wasted in the effluent. At an approximately one-to-one substrate COD ratio, n-caprylate production stopped in the bioreactor. The experimental results in this study generally backed up the predictions of a generalized thermodynamic model we developed to predict the thermodynamic feasibility of n-caprylate production at different ethanol-to-acetate ratios. Finally, Illumina 16S rRNA gene sequencing of the bioreactor microbiome across time and in two sampling locations on the bioreactor revealed a relatively uneven microbiome that was dominated by Firmicutes and Proteobacteria phyla members.